Geology Reference
In-Depth Information
Crashing together: Convergent plate boundaries
When two crustal plates move toward each other, they create a convergent plate bound-
ary. At a convergent boundary, part of the lithosphere is forced downward into the
mantle while other parts of it may be forced upward, building mountains. Exactly how
this process occurs, and to what degree, depends on the type (and therefore density) of
the crustal plates.
Three convergent boundary types are possible, as I describe in this section.
One goes up and one must go down
The most common convergent plate boundary, and the easiest to recognize, is one
where a plate of continental crust meets a plate of oceanic crust. When this occurs, the
more dense oceanic crustal plate is forced downward beneath the continental crustal
plate in a process called
subduction.
Regions where this occurs are called
subduction
zones.
Most subduction zones share certain features, such as a chain of volcanoes and
earthquakes in a pattern of gradually deepening locations as illustrated in Figure 9-4.
Figure 9-4:
A
continental-ocean-
ic plate conver-
gent boundary
subduction zone
and associated
geologic features.
As the dense oceanic plate is forced downward, or
subducted,
it rubs against the contin-
ental plate, causing friction that creates heat. This heating due to friction causes the
subducting crustal rocks to melt, becoming a liquid (magma). The density of the rock
materials decreases dramatically after they are melted. The molten rock material then
begins to move upward through the continental crustal rocks of the overlying plate until
it erupts onto the surface.
